1. Field of the Invention
The invention relates to a braking force control device and a braking force control method and, more particularly to a braking force control device and a braking force control method for generating a braking force greater than usual, when an emergency brake operation is executed.
2. Description of Related Art
JP(P) 9-290743 discloses a braking force control device capable of executing a brake assist control generating a braking force greater than usual, when the emergency brake operation is executed. In this conventional braking force control device it is determined that the emergency brake operation is executed when the master cylinder pressure becomes equal to or greater than a first threshold value, and the elevation gradient of the master cylinder pressure becomes equal to or greater than a second threshold value. Such a determination method prevents the brake assist control from being initiated improperly by an erroneous determination of an emergency brake operation when, for instance, the brake is operated rapidly quite temporarily, or the brake is operated successively with a large pedal depression force during high speed driving.
In general, the braking device comprises a brake booster having a function to generate a great braking force by assisting the brake operation. The brake booster assists the brake operation by using as a power source, the negative pressure supplied from the engine inlet pipe, for generating a large master cylinder pressure. However, the inlet pipe negative pressure varies according to the engine speed, accelerator opening or other vehicle driving conditions. Consequently, if the inlet pipe negative pressure drops and the negative pressure supplied to the brake booster becomes insufficient, the brake operation can not be assisted sufficiently. Under such condition, when the brake is operated, the start-up of master cylinder pressure may be delayed, compared to the case where enough negative pressure is secured for the brake booster. In this case, when the emergency brake is operated, namely, when the brake pedal is depressed by the driver with a predetermined pedal depression force and a predetermined speed, the master cylinder pressure elevation gradient becomes once equal to or greater than the second threshold value; however, when the master cylinder pressure becomes equal to or greater than the aforementioned first threshold value, the master cylinder pressure elevation gradient might be less than the second threshold value. Therefore, in the emergency brake operation determination by the aforementioned conventional braking force control device, when the brake booster negative pressure lowers, even if the driver executes the emergency brake operation, it may not be determined as an emergency brake operation, and therefore the brake assist control may not be executed.
Moreover, the master cylinder pressure response characteristics to the brake operation are different according to brake systems specifications; including not only the brake booster, but also the wheel cylinder size, oil consumption during the braking operation or the like. Thus, the first threshold value or the second threshold value should be set for each system (each model), deteriorating the universality of the system.